Clarksville is the county seat of Montgomery County, Tennessee, United States. It is the fifth-most populous city in the state, after Nashville, M…Clarksville is the county seat of Montgomery County, Tennessee, United States. It is the fifth-most populous city in the state, after Nashville, Memphis, Knoxville, and Chattanooga. The city had a population of 166,722 as of the 2020 census.